Attempt refresh of neigh table on miss

When an address is new it may not be in the last
captured neighbor table. Induce refresh before deciding
that neighbor is unavailable.

confluent_server/confluent/discovery/protocols/slp.py

@@ -107,6 +107,8 @@ def _parse_slp_packet(packet, peer, rsps, xidmap):
     if '%' in addr:
         addr = addr[:addr.index('%')]
     mac = None
+    if addr not in neighutil.neightable:
+        neighutil.update_neigh()
     if addr in neighutil.neightable:
         identifier = neighutil.neightable[addr]
         mac = identifier